1. Technical Field
The present invention relates generally to hydrocarbon recovery. More particularly, the present invention relates to a process for improving conformance in a subterranean formation from which hydrocarbons are to be recovered. The present invention particularly, though not exclusively, relates to a process utilizing a crosslinked polymer gel to improve conformance in a subterranean formation from which hydrocarbons are to be recovered.
2. Description of Related Art
The injection of displacement fluids into subterranean hydrocarbon-bearing formations to promote the production of hydrocarbons therefrom is well known. Water and various gases, in addition to more complex fluids, such as surfactant solutions and polymer solutions, are common displacement fluids utilized in both miscible and immiscible oil displacement floods.
The effectiveness of oil displacement floods is reduced by conformance problems in the formation. Conformance problems are generally characterized by the juxtaposition of high permeability and low permeability flow paths within a given formation. Conformance problems can be either fracture-type or matrix-type. Fracture-type conformance problems occur when there are simple fractures or fracture networks in communication with an injection and/or production well penetrating the formation. Matrix-type conformance problems occur when there are adjoining strata or regions of differing permeability within the formation.
When a displacement fluid is injected into a formation exhibiting conformance problems, the high permeability flow paths divert substantially all of the displacement fluid away from the low permeability flow paths. As a consequence, the displacement fluid does not sweep the low permeability flow paths and the sweep efficiency of the displacement fluid in the formation is poor.
Conformance problems can be corrected by a conformance improvement treatment (CIT) which effectively plugs or constricts the high permeability flow paths with a plugging material. By plugging or constricting formation flow paths having high permeability and low oil saturation, subsequently injected displacement fluid preferentially sweeps formation flow paths having low permeability and high oil saturation. Thus, the CIT improves the sweep efficiency of the displacement fluid, enables it to contact and displace more oil, and promotes increased incremental oil recovery.
At present, gels are commonly used as a CIT plugging material. Polyacrylamide crosslinked with chromium (III) has been found to form gels which are effective for most CIT's as set forth in U.S. Pat. No. 4,683,949 to Sydansk et al. U.S. Pat. No. 4,683,949 describes the effective use of crosslinked acrylamide gels specifically made for treatment of fracture-type and matrix-type conformance problems. It has, however, been found that in some cases conventional gels employed in CIT's are ineffective for their intended purpose because in situ gelation rates of the polymers cannot be satisfactorily controlled.
In particular, although acrylamide polymers have been found to produce satisfactory gels for CIT's, under certain formation conditions such as low temperatures partially hydrolyzed polyacrylamides with more than about 1% of the amide groups prehydrolyzed to carboxylate groups have been found to gel too rapidly using a trivalent chromium crosslinking agent. Conversely, unhydrolyzed polyacrylamides with less than about 0.1% of the amide groups prehydrolyzed to carboxylate groups have been found to gel too slowly using trivalent chromium because very little in situ amide hydrolysis occurs at low temperatures. Adjustment of the polyacrylamide carboxylate concentration to a range between about 0.1 to about 1% appears to be ideal for low temperature applications, but in practice it is extremely difficult to obtain and subsequently maintain polymers within this carboxylate concentration range. Furthermore, the strength of mature gels resulting from such polymers is often too weak for the desired hydrocarbon recovery application.
It is apparent that the initial polymer carboxylate concentration is not a suitable variable under low temperature conditions for controlling the gelation rate of a gelation system. Modification of the gelling agent is often used as an alternative means to control gelation rate, but this can also have undesirable consequences on the gelation system which are not foreseeable. Accordingly, it is more desirable to achieve gelation rate control by adjusting other less sensitive gelation parameters.
Thus, a CIT is needed utilizing a gelation system which gels in situ at a relatively rapid yet controlled rate enabling adequate placement of the gel in the treatment zone. Further, a CIT is needed which enables relatively simple adjustment of the gelation system to effect corresponding corrections in the gelation rate without deleteriously altering the overall performance of the gelation system and the gel resulting therefrom.